(1) Field of the Invention
The present invention relates to multimode hydrophones for receiving acoustic signals and more specifically to a small diameter, low frequency multimode hydrophone which provides direction finding capabilities for locating an acoustic source over a decade band of low frequencies.
(2) Description of the Prior Art
Presently, at frequencies below 1.0 kHz, pressure gradient hydrophones, generally with dimensions less than one-sixth of a wavelength at the highest frequency of interest, are used as sensors for direction finding systems. At frequencies above 1.0 kHz, multimode hydrophones are used which are thin wall tubes divided electrically into quadrants to permit the differencing of opposing halves to produce signals with dipole directivity patterns and the formation of a nondirectional reference signal by summing all four quandrants. Such hydrophones use a plurality of modes of vibration, specifically the first circumferential mode from which two orthogonal dipole patterns can be derived for direction finding and also the radial mode which serves as a reference signal to permit removal of directional ambiguities or which, when added to or subtracted from the dipole signals, using proper gain and phase compensation, will yield a cardoid directivity pattern.
Multimode hydrophones can operate satisfactorily over a decade of frequency with the range of frequencies determined by hydrophone diameter. The performance of conventional piezoelectric and magnetostrictive multimode units is satisfactory when the hydrophone diameter is greater than one-tenth of a wavelength but less than one wavelength. In order to shift the usable operating range of frequencies lower, it is necessary to increase the diameter of the tube, e.g., at 1.0 kHz, the tube diameter would have to be at least 5.0 inches or larger to provide satisfactory dipole directivity patterns. For many hydrophone applications such a large diameter is unacceptable in that thin wall tubes of large diameter are not manufacturable as a single piece of ceramic; also, for applications such as dome mounting or towed array use, minimizing flow induced noise by using small diameters is crucial to satisfactory operation especially as pertains to low frequency directional applications.
What is thus required is a small diameter multimode hydrophone which operates over a decade range of low frequencies.